Electronic data systems are frequently interconnected using network communication systems. Area-wide networks and channels are two approaches that have been developed for computer network architectures. Traditional networks (e.g., LAN's and WAN's) offer a great deal of flexibility and relatively large distance capabilities. Channels, such as the Enterprise System Connection (ESCON) and the Small Computer System Interface (SCSI), have been developed for high performance and reliability. Channels typically use dedicated short-distance connections between computers or between computers and peripherals.
Features of both channels and networks have been incorporated into a new network standard known as "Fibre Channel". Fibre Channel systems combine the speed and reliability of channels with the flexibility and connectivity of networks. Fibre Channel products currently can run at very high data rates, such as 1062 Mbps. These speeds are sufficient to handle quite demanding applications, such as uncompressed, full motion, high-quality video. ANSI specifications, such as X3.230-1994, define the Fibre Channel network. This specification distributes Fibre Channel functions among five layers. The five functional layers of the Fibre Channel are: FC-0--the physical media layer; FC-1--the coding and encoding layer; FC-2--the actual transport mechanism, including the framing protocol and flow control between nodes; FC-3--the common services layer; and FC-4--the upper layer protocol.
There are generally three ways to deploy a Fibre Channel network: simple point-to-point connections; arbitrated loops; and switched fabrics. The simplest topology is the point-to-point configuration, which simply connects any two Fibre Channel systems directly. Arbitrated loops are Fibre Channel ring connections that provide shared access to bandwidth via arbitration. Switched Fibre Channel networks, called "fabrics", are a form of cross-point switching.
Conventional Fibre Channel Arbitrated Loop ("FC-AL") protocols provide for loop functionality in the interconnection of devices or loop segments through node ports. However, direct interconnection of node ports is problematic in that a failure at one node port in a loop typically causes the failure of the entire loop. This difficulty is overcome in conventional Fibre Channel technology through the use of hubs. Hubs include a number of hub ports interconnected in a loop topology. Node ports are connected to hub ports, forming a star topology with the hub at the center. Hub ports which are not connected to node ports or which are connected to failed node ports are bypassed. In this way, the loop is maintained despite removal or failure of node ports.
More particularly, an FC-AL network is typically composed of two or more node ports linked together in a loop configuration forming a single data path. Such a configuration of a node port-to-node port loop is shown in FIG. 1. In FIG. 1, six node ports 102, 104, 106, 108, 110, 112 are linked together by data channels 114, 116, 118, 120, 122, 124. In this way, a loop is created with a datapath from node port 102 to node port 104 through data channel 114 then from node port 104 to node port 106 through data channel 116, and so on to node port 102 through data channel 124.
When there is a failure at any point in the loop, the loop datapath is broken and all communication on the loop halts. For example, if node port 104 fails, data no longer passes through node port 104. A failure may also occur in a data channel between node ports, such as by a physical break in the wire or electromagnetic interference causing significant data corruption or loss at that point. At this point, loop 100 has been broken. Data no longer flows in a circular path and the node ports are no longer connected to one another. The loop has, in effect, become a unidirectional linked list of node ports.
A conventional technique to avoid broken datapaths in a node port-to-node port loop introduces a hub within a loop. A hub creates a physical configuration of node ports in a star pattern, but the virtual operation of the node ports continues in a loop pattern. The connection process (i.e., sending data between node ports) and interaction with the hubs is effectively transparent to the node ports connected to the hub, which perceive the relationship as a standard Fibre Channel arbitrated loop configuration.
FIG. 2 illustrates an arbitrated loop 200 with a centrally connected hub. Similar to loop 100 illustrated in FIG. 1, loop 200 includes six node ports 202, 204, 206, 208, 210, 212, each attached to a hub 214. Hub 214 includes six hub ports 216, 218, 220, 222, 224, 226 where each hub port is connected to another hub port in a loop topology by a sequence of internal hub links. In this way, node ports 202-212 are each connected to a corresponding hub port 216-226. Thus, node ports 202-212 operate as though connected in a loop fashion as illustrated in FIG. 1. Data typically flows into a hub port from an upstream hub port, into the attached node port, back from the node port to the hub port, and out of the hub port to a downstream hub port.
When a node port or a data channel fails or is disconnected, the loop is maintained by bypassing the failed node port. In a conventional hub, when a hub port no longer receives data from a node port, the hub port goes into a bypass mode. In bypass mode, rather than passing data received on the data channel from the node port, the hub port passes data received along the internal hub link from the previous, upstream hub port Thus, nodes are removed and inserted in the loop by changing the corresponding hub port in and out of bypass mode.
The content of a datasteam of an FC-AL network is defined by FC-AL protocols. Characters are constantly moving through the loop from one port to the next. These characters may be actual data or loop control signals. Loop control signals are always present in the datastream except when a data frame is being sent from a source node port to a destination node port. Under FC-AL protocols, the loop control signals are ordered sets, including primitive signals and primitive sequences. Ordered sets typically begin with a special character indicating the beginning of an ordered set, such as K28.5.
A data frame includes a series of one or more data words preceded by a frame initiation primitive and followed by a frame termination primitive. An FC-AL data frame includes an uninterrupted stream of data preceded by a special ordered set called a Start Of Frame ("SOF") and succeeded by a special ordered set called an End Of Frame ("EOF"). An End Of Frame Abort ("EOFA") is a special type of EOF ordered set, for aborting a frame. An interframe gap occurs after a frame termination primitive and before the next frame initiation primitive. Under FC-AL protocols, an interframe gap is defined to include six ordered sets, by default.
A datastream of encoded characters ideally always has a valid "running disparity". The encoded characters are defined according to a conventional 8B/10B encoding scheme, defined in Fibre Channel protocols. The running disparity at the end of a character in the datastream is the difference between the number of 1's and 0's in the bit encoding of the character. A character with more 1's than 0's has a positive running disparity. A character with more 0's than 1's has a negative running disparity. A character with an equal number of 1's and 0's has a neutral running disparity. An encoder transmits a positive, negative, or neutral disparity encoded character. A neutral character does not affect the running disparity of the datastream. A positive character changes the running disparity from negative to positive and a negative character changes the running disparity from positive to negative.
Each word has an overall running disparity as well. The running disparity for a word determines the effect that word has on the running disparity of the datastream. As with characters, a word with a positive running disparity changes the running disparity to positive at the end of the word. Similarly, a word with a negative disparity changes the running disparity to negative and a word with a neutral disparity leaves the running disparity the same as the running disparity before the word.
The running disparity between the words that form the interframe gap is defined to be negative. The last word of a frame, an EOF, ensures that the running disparity is negative. Each ordered set in the gap between frames has an overall running disparity of neutral so that the running disparity at the end of each word remains negative.
If the encoder sends a negative disparity encoded character when the running disparity is negative, or a positive encoded character when the running disparity is positive, a running disparity error results. This error typically introduces an invalid character into the loop.
Ideally, all the data in the loop are valid data characters, all the control signals are valid ordered sets, there are no running disparity errors, data is properly formatted into data frames, and only ordered sets are present between data frames. However, errors are sometimes introduced into loops for a variety of reasons, such as when devices are inserted into the loop, bad cables are used, or when a device does not comply with FC-AL protocols. These errors can create invalid characters and hence invalid transmission words inside of data frames. Invalid transmission words occur when: a word, either a data word or an ordered set, includes an invalid character; an ordered set does not have the correct beginning running disparity; or, a word includes a misaligned special character.
The conventional solution upon receiving an invalid transmission word is to replace the invalid character in the invalid tranission word with a valid transmission character. This replacement typically occurs when the invalid word is decoded. The decoder generates a valid word by replacing the invalid character in the received word with a pseudo-random valid character. Hence, this replacement simply "hides" an error in a seemingly valid word. In addition, this replacement does not correct for an invalid transmission word caused by invalid special character alignment. Because these errors remain uncorrected, they are also in effect hidden. Thus, data frames which include hidden errors can propagate throughout the loop. The downstream ports do not have any indication that the frame was received in error. When the seemingly valid word is later encoded at a downstream hub port, the hidden error is included in the encoded word, further compounding the error. Because of these hidden errors, the destination node needs to check the CRC (i.e., cyclical redundancy checking) code for each frame to ensure that the frame does not include such hidden errors.
The inventor has determined that it would be desirable to provide apparatus and methods for clearly identifying frames which are received in error and avoiding the propagation of data frames including hidden errors.